<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            display: flex;
            width: 800px;
            height: 300px;
            background-color: pink;
            flex-direction: row;
            justify-content: center;
            /* 水平垂直居中 */
            /* align-items: center; */
            /* align-items: flex-start; */
            /* 子元素不设置高度时，会向侧轴方向延伸 */
            align-items: stretch;
        }

        div span {
            width: 150px;
            /* height: 100px; */
            background-color: skyblue;
        }
    </style>
</head>

<body>
    <div>
        <span>1</span>
        <span>2</span>
        <span>3</span>
    </div>
</body>

</html>